1. Field of Invention
The present invention relates to coating pigments and fillers. In particular, the present invention concerns novel polysaccharide-based pigments and fillers and methods of preparing such products.
2. Description of Related Art
Coating pigments and fillers are used in papermaking for improving smoothness, brightness and opacity of fibre-based products. Reduced grammage of the paper products and the relative low cost of mineral components has led to increasingly larger proportions of inorganic materials being incorporated into the fibre-based products to maintain and improve the optical properties. Today, printing paper and board grades therefore contain considerable amounts of inorganic pigments and fillers. However, the inorganic (mineral) pigments increase the proportion of incombustible residues and impair the recyclability of the products in the deinking stage, in combustion and in composting.
For replacing mineral products as fillers and pigments, various synthetic, more easily combustible organic pigments have been suggested and provided. They are conventionally based on petroleum derivatives and are solely used in coatings mixed with inorganic pigment. Also some organic, modified polysaccharide-based pigments and fillers have earlier been proposed. In this respect, reference is made to the following published International Patent Applications: WO2005030844, WO2005030847, WO2006134211, WO2007003697, WO2007003699, WO2007082985.
The particular advantages of polysaccharide-based (e.g. starch-based) pigments and fillers are that they are based on a renewable raw-material source and can easily be recycled.
As noted above, pigments and fillers are often used for improving brightness and opacity. The optical properties and especially the light scattering capacity of the paper structure are affected by the size, shape, packing and optical character of the fibrous components, the pigmenting components and pores. Light reflects partially from surfaces that differ from their surroundings by their refractive index (RI). The refractive index is a fundamental physical constant related to the molecular structure, density and atomic polarizability of a material and describes the deceleration of the speed of light propagating in the material. Also the light absorption character of the material affects the refractive index.
Thus, in the structure of the papermaking fibres, or in a coating, improved light scattering potential is typically associated with pigmenting materials that possess higher values of refractive index than that of the fibres (and air in the pores).
The inorganic materials commonly used have a high refractive index (commonly designated the symbol “n”) and, in particular, titanium dioxide (rutile) has an extremely high n-value of 2.76, whereas gypsum, kaolin, calcium carbonate and talc have n-values in the range from about 1.54 to 1.59. By contrast, the refractive indices of organic pigmenting materials inherently are close to that of cellulosic papermaking fibres, i.e. close to, but below, 1.5.
Based on the above, in view of the considerable advantages provided by organic particles based on renewable sources, there is a need for new polysaccharide-based pigments and fillers which have improved optical properties.